The use of printed security indicia on identity documents is known in the industry. For example, the production of latent optically encoded images comprising overlying line deflection patterns is disclosed in each of Canadian Patent Nos. 1,172,282 to Trevor Merry and 2,071,795 to Trevor Merry et al. Another example is disclosed in U.S. Pat. No. 5,708,717 to Alfred Alasia according to which a visible source image and latent image are optically scrambled by means of a computer to produce a combined image of which only the source image is visible to the unaided eye but the latent image may be identified by applying an optical lens to the combined image. In all of these examples the latent (hidden) image is detected by means of an optical decoder comprising lenticular lenses in a physical or computerized form (i.e. through the use of either a lenticular finding screen or, possibly, a digitally represented lens pattern overlay onto a computer display). However, such latent optically encoded images provide to a document only a single level of secured identifying indicia, with the security level being determined by the optical encoding parameters.
The scope of the parameters (variables) which may be used to optically encode an image, such as in the foregoing encoding methods, is relatively narrow and limited by the physical specifications of both the printing method used to print the latent optically encoded image onto the document and the optical and physical manufacturing limitations of the lenticular finding screens which are needed to decode such latent images. Consequently, the degree of security provided by optical coding methods alone is less than the high level of security which may be required for many applications.
Therefore, there is a need in the marketplace, and the security printing industry, for an image encoding/decoding system which is able to provide different levels of security by a single printed security indicia applied to a document including a high level of security.